1. Field of the Invention
This invention relates to a novel graft copolymer and more particularly to a polyvinyl acetal resin grafted with an organopolysiloxane. The invention also relates to a coating composition comprising the novel graft copolymer.
2. Description of the Prior Art
Polyvinyl acetal resin is a general term for polyvinyl butyral resins and polyvinyl formal resins, and has features such as (1) capability of forming a flexible film, (2) good adhesion properties, and (3) good dispersability of pigments. Accordingly, the resin has wide utility in the fields of coating binders, ink bases, wash primers, covers for electric wire, general-purpose adhesives, adhesives for printed board, and the like. For the purpose of imparting releasing properties, slip properties, water repellency and the like specific properties to the polyvinyl acetal resin, attempts have been made to introduce silicones into the resin.
However, dimethyl silicone oils are not miscible with polyvinyl acetal resins. When the oil is added to the polyvinyl acetal resin, the resultant film will inconveniently become opaque and the surface properties will become poor. If silicone oils whose miscibility with the resin has been improved by modification with a phenyl group, a long-chain alkyl group, a polyoxyalkyene or the like is added, the above inconvenience may be overcome but with the disadvantage that the releasing properties, slip properties and water repellency do not continue long. To avoid this, attempts have been made to introduce silicone oils having reactive group, such as --NCO, --COOH, ##STR1## or --SiCl, which are reactive with the hydroxyl groups of the polyvinyl acetal. However, problems occur with the shelf life, and the addition of catalysts or the use of heat is undesirably necessary, thus placing a limit on application.